1. Field of the Invention
This invention relates to a focusing arrangement for a video camera, and especially to an arrangement for aiding manual focusing through an electronic viewfinder. More specifically, the invention pertains to a signal processing circuit of the type that isolates a focus-related characteristic of a video signal and uses the isolated characteristic to accentuate the display of a well-focused image in the viewfinder.
2. Description Relative to the Prior Art
In a manually focussed video camera, an electronic viewfinder is not only used to compose a scene as recorded by the camera but also to determine when the scene image is properly focused. As is the case with many optical systems, the proper focus is not a discrete distance but a range of distances through which the image is acceptably defined, i.e., a range indicated by the depth of field for a particular optical (lens) aperture and subject distance. An electronic viewfinder repeats images of the scene at the television frame rate, i.e., 1/30 second. This "exposure interval" dictates a particular lens aperture for the existing light condition and, therefore, establishes a particular depth of field for each subject distance. A special problem emerges when the viewfinder is used with a video still camera, which provides several exposure interval and aperture combinations for each light condition. The light condition that dictates a certain aperture . . . and thus a certain depth of field . . . for the electronic viewfinder may require an entirely different aperture for the still exposure (because the exposure interval may not be 1/30 second). Thus the depth of field for viewing may be an artificial indication of the actual picture-taking condition. The "real" depth of field may be considerably compressed from that observed in the viewfinder with the attendant possibility that a properly-focused viewfinder display may be unfocused with respect to the recorded picture.
If one could reliably locate the central focus position for the displayed depth of field, the picture would be in focus for any aperture (that is, for any depth of field). Owing, however, to the small size of the display screen in the viewfinder and the limited bandwidth available for the display, differences between details shown on the viewfinder screen are not very sharp or clear. Searching for optimum focus, necessary in view of the depth of field considerations heretofore mentioned, is a procedure marked by uncertainty. U.S. Pat. No. 4,481,540 suggests one arrangement for dealing with such focus problems. Two versions of the video signal, one unmodified and the other blurred by a low pass filter, are applied to the viewfinder via a switching device controlled by the high frequency content of the video signal. The unmodified part of the image is switched to the display during the presence of high frequency content while the blurred part of the image is displayed otherwise. As the image is brought into focus, more unmodified image, and less of the intentionally blurred image, appears in the viewfinder.
The focusing arrangement in U.S. Pat. No. 4,481,540, however, requires a special viewing mode that detracts from the other purpose of the viewfinder . . . to observe the scene as recorded by the camera. Secondly, the focused image, which is scattered throughout the field of view, requires the viewer to key upon the whole picture rather than to concentrate attention upon some unshifting area. But most importantly, the relatively low bandwidth of the electronic viewfinder ordinarily limits the rendition of high frequency, well-focused picture information. The difference between the blurred and the unmodified but focused image is ordinarily not great enough to be eye-catching.